CHAPTER 13

Design a URL Shortener

A deceptively simple read-heavy system: unique keys, redirects, caching, analytics

Source: PDF p. 49

Core PDF content

Each case study in this part follows the same template: requirements, estimation, API and data model, high-level design, deep dives, and trade-offs. Use it as your interview skeleton.

Functional requirements	Non-functional requirements and scale
- Create a short URL for a long URL (optional custom alias, expiry) - Redirect short URL to the original - Basic click analytics - Delete / expire links	- Very high availability for redirects (99.99%), p99 < 100 ms - Short links must not be guessable or collide - 500 M new links/month, read:write = 100:1 - Links stored for 5+ years

Quantity	Estimate	Design implication
Write QPS	500 M / (30 x 86,400) = ~200/s (peak ~600)	Trivial for one DB; scale is in reads
Read QPS	200 x 100 = 20,000/s (peak ~60,000)	Cache hot links; replicas; CDN optional
Total records (5 years)	500 M x 12 x 5 = 30 B	Needs sharding or a scalable KV store
Storage	30 B x ~500 B = ~15 TB	Fits a modest sharded cluster
Code length	62^7 = 3.5 T > 30 B	7 base-62 characters are enough

API and data model p. 49

POST       /v1/urls
                 {"long_url": "...", "custom_alias": "?", "expires_at": "?"}
                    -> 201 {"short_url": "https://sho.rt/aZ3kP9q"}
GET    /{code}      -> 302 Location: <long_url>

DELETE /v1/urls/{code} -> 204

Table urls(code PK, long_url, user_id, created_at, expires_at). Access is purely by key, so a key-value / wide-column store (DynamoDB, Cassandra) or a sharded SQL table keyed by hash(code) fits. Click events go to a stream (Kafka), never to the redirect path.

Figure 34: URL shortener: the write path creates keys; the read path is a cache-first lookup, and analytics are decoupled through a stream. — Figure 34. URL shortener: the write path creates keys; the read path is a cache-first lookup, and analytics are decoupled through a stream. · PDF p. 49

Review questions for this chapter have not been written and reviewed yet. The chapter text above is the full source content.

Try it

Open “URL shortener” in Labs

References

System Design & AI System Design: The Complete Guide — 2026 Edition, p. 49; figures 34.

Ch. 12 Ch. 14: Design a Distributed Key-Value Store

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